import sys
import os


def QueryYesNo(question, default=None):
    valid = {"yes": True, "y": True, "ye": True,
             "no": False, "n": False}
    if default is None:
        prompt = " [y/n] "
    elif default == "yes":
        prompt = " [Y/n] "
    elif default == "no":
        prompt = " [y/N] "
    else:
        raise ValueError("invalid default answer: '%s'" % default)

    if 'EYEOS_ALWAYS_YES' in os.environ:
        return valid['y']
    else:
        while True:
            sys.stdout.write(question + prompt)
            choice = input().lower()
            if default is not None and choice == '':
                return valid[default]
            elif choice in valid:
                return valid[choice]
            else:
                sys.stdout.write("Please respond with 'yes' or 'no' "
                                 "(or 'y' or 'n').\n")
